In the field of cellular communications, mobile subscriber units located in one cell communicate with a base station over a first set of communication channels. In adjacent cells, mobile subscriber units communicate with base stations over different sets of communication channels. When, during the course of a call, movement causes a mobile subscriber unit to cross a boundary between cells, the call is "handed-off" from a communication channel associated with one cell to a communication channel associated with another cell. The hand-off operation requires base stations to alter their allocation of communication channel resources and causes the mobile subscriber unit to retune its transmitter and receiver to a new channel while the call is ongoing. Desirably, these operations are carried out without the call being dropped or otherwise interrupted and without unnecessarily wasting scarce communication channel resources.
The hand-off process has conventionally been a delicate operation which has undergone continuous refinement. The hand-off process includes the creation and implementation of rules which are followed in deciding when to perform a hand-off, selecting which communication channel can receive a handed-off call, and successfully transferring an ongoing call between communication channels. Some aspects of the continuous refinement result from an ongoing desire to enhance system performance.
However, other aspects of the continuous refinement result from inevitable changes which take place in the overall cellular system structure from time to time. For example, a frequency reuse plan may change from time to time, redefining an existing association of communication channels to cells. The hand-off process often needs to evolve to reflect the change.
Due to a continuing need for refinement and a need to provide consistently good service, conventional cellular systems implement their hand-off processes centrally under the control of the communication service providers. Mobile subscriber units are typically not under the communication service providers' control and lack the information which drives hand-off decision rule evolution. Thus, mobile subscriber units conventionally play only a minor role in the hand-off process. For example, a mobile subscriber unit may, when instructed by a base station, make signal strength measurements and report the measurements back to the base station. However, the base station and other system components follow hand-off decision rules and instruct the mobile subscriber unit to retune its transmitter and receiver at a time and to a channel dictated by the decision rules.
In the face of an upcoming cellular system structural change, such as a change in the frequency reuse plan, system components and not mobile subscriber units have a priori knowledge of the changes and can modify decision rules to minimize call droppage which might otherwise occur due to hand-offs performed in accordance with stale rules. Alternately, if such changes occur rarely, a cellular system may simply tolerate operation for a short duration on stale decision rules, but schedule the change to occur when massive numbers of dropped calls will be avoided, such as in the middle of the night.
Unfortunately, conventional "centralized" hand-off techniques are unsuitable for cellular systems in which base stations are located in satellites orbiting the earth. Enormous costs are associated with placing a satellite in orbit, and base stations that are not simple, light-weight, highly reliable, and low-power are not commercially competitive. Conventional centralized hand-off techniques prevent base stations from having these characteristics due, at least in part, to the need to incorporate sufficient processing power to implement hand-off decision rules for all channels supported by the base stations.